1. The Field of the Invention
The present invention relates to processes for the removal of native oxide from polysilicon and other silicon containing layers in the construction of integrated circuits. More particularly, the present invention is directed to a process of integrated circuit construction in which native oxide is first removed through a fluorine containing cleaning procedure followed by the further removal of residues resulting from the fluorine containing cleaning procedure.
2. The Relevant Technology
In the manufacture of integrated circuits, structures often need to be formed whereby a previously deposited silicon surface is covered with a later deposited conducting layer. One difficulty in producing these structures arises from native oxides which grow on the silicon surface previous to the deposition of the conducting layer. Native oxides, such as silicon dioxide, are formed from the reaction of silicon and oxygen, and occur whenever silicon is exposed to an oxygen containing ambient. Native oxides are typically insulative and highly resistive. Thus, when a secondary layer to be formed above the silicon is intended to form a conductive interface with the silicon, the native oxide can inhibit a fully conductive interface.
This problem is typically encountered when depositing a metal over an underlying silicon layer. One example is the formation of tungsten silicide, which is useful when forming MOS technology gate structures. When so doing, typically a layer of tungsten is deposited by chemical vapor deposition over a layer of polysilicon and annealed to form tungsten silicide. As discussed, the difficulty in this process occurs from the growth of native oxide atop the polysilicon after the polysilicon has been deposited. The native layer must be removed in order to properly form the tungsten silicide structure and to adhere it strongly to the polysilicon underlayer. Failure to do so will result in the native oxide layer acting as a resistive barrier between the polysilicon and the tungsten and prohibiting a proper anneal.
One method previously used for removing native oxide films from polysilicon and silicon substrates comprises the use of hydrofluoric acid. Typically, the silicon layer, in this case polysilicon, is exposed to the hydrofluoric acid vapor clean in a first chamber and is then transferred thereafter to the deposition chamber for deposition of tungsten. This transfer preferably occurs in a vacuum in order to avoid further native oxide growth. An undesirable side effect of this problem is the formation of a visual haze on the silicon or polysilicon. The hydrofluoric acid clean typically results in a residue of fluorine ions which form nucleation points for the formation of silicon tetrafluoride (SiF.sub.4) particulate matter. This SiF.sub.4 particulate matter forms the haze.
If the wafer on which the silicon substrate is formed is transferred directly to the deposition chamber with no further cleaning processes, the SiF.sub.4 or SiF.sub.6 particulate matter causes defects that impair the adhesion of the tungsten silicide film to the polysilicon. The consequence is that the tungsten film will form a poor interface and may even totally lift off of the substrate. Furthermore, a proper anneal can also be obstructed, resulting in the tungsten silicide not being formed and a consequent high resistivity of the structure. A similar effect would occur with the interface of any metal to a silicon substrate.
A remedy to this problematic effect in the art has been to water clean the wafer in a water bath after the hydrofluoric clean. A typical example is a vapor etch process performed in an Excalibur etching machine, manufactured by FS1 located in Chaska, Minn., U.S.A. Water clean processes must necessarily be undertaken in a bath chamber, after which the wafers are removed and transferred to a tungsten deposition chamber. This removal and transfer of the wafers is undesirable as it results in further exposure to the ambient (i.e. air) and consequently to further native oxide growth. Also, such processes do not always fully rid the surface of the SiF.sub.4 particulate matter. Thus, the prior art processes are not fully satisfactory.
As a result of the problems discussed above, it is evident that a need exists in the art for a process of cleaning native oxide layers from silicon substrates in preparation for the subsequent deposition of a conducting layer. Such a process preferably could utilize fluorine cleaning processes without leaving behind SiF.sub.4 particulate matter. Finally, such a process should also be able to be performed in a non-oxidizing ambient, such as performing the process within a cluster tool.